Expansion of neutron star matter and its final nuclear composition

The final nuclear composition of the matter expanding from the density of a neutron star is investigated. It is assumed that starquakes cause the cracks which penetrate the neutron star crust and that the neutron star fluid can flow out through the cracks into space. The change with time of the nuclear composition of this matter is calculated by use of the compressible nuclear mass formula, and the hydrodynamics of the system is followed by the effect of nuclear transformation with time. It becomes clear that the temperature of the system decreases monotonously up to the time of the second fission of heavy neutron-rich nuclei, which is followed by a rapid rise to above 109 K. If the value of the β-strength function exceeds about 10sup(-5,5) MeV-1 s-1, the system proceeds to a state of nuclear equilibrium in the later expansion stage and the nuclear composition is reshuffled, finally to be transformed into neutron-excess, stable nuclei within the atomic mass region 80 < approx. A < approx. 120. It also becomes clear that if the strength function has a value smaller than the above critical value, then the neutron-rich nuclides with A element of [200, 400] are copiously produced. These results will also be applied in the cases of a neutron-star - black-hole collision and the explosion of a neutron star associated with the catastrophic phase transition within the neutron star core. The astrophysical implications are briefly discussed. (orig.)